This section provides background information related to the present disclosure which is not necessarily prior art.
A transformer is a device that transfers electrical energy from one circuit to another through inductively coupled conductors. The inductively coupled conductors are the transformer's coils or windings.
In one form, a transformer has two galvanically separated coils. These coils are commonly referred to as a primary winding and a secondary winding. Designation as the primary winding is usually given to the winding that is galvanically connected to a source of energy or circuitry actively controlling electrical parameters. The secondary winding is typically the winding that is connected to a receiver of energy or a circuit passively responding to the actions of the primary circuitry. Of course, primary/secondary designations are typically not meaningful with respect to the transformer itself and are descriptive only for the role this transformer performs in the overall circuit. Primary and secondary windings work the same way as to the main principles of transformers. With a transformer with identical primary and secondary coils, for example, the coils can be interchanged without any impact on the operation of a circuit (or circuits) connected to such transformer. Interchanging the coils of a transformer having different primary and secondary coils would change voltage and current relationships, but would impact connected circuitry only, while the transformer itself would work the same way. Furthermore, the primary and secondary windings may be connected, used, etc. in ways other than common transformers, rendering the primary and secondary terminology meaningless (and possibly confusing). Terminology becomes even more confusing with transformers having multiple windings, including, for example, magnetic structures as disclosed in the present application. Therefore, numerical designations for various windings (instead of primary-secondary) will typically be used herein.
FIG. 1 illustrate a two winding transformer, generally indicated by the reference numeral 100, along with the voltages V1, V2 across the windings of the transformer 100 and the currents I1, I2 through the windings of the transformer 100. To improve energy transfer between windings, a highly magnetic (high permeability) material is commonly used as a transformer core 102. This core 102 provides a low reluctance path for the magnetic field, passing through both windings, such that nearly all of the magnetic field is enclosed by the first and second coils. The relationship between voltages and currents in a two winding transformer (e.g., transformer 100) are determined by the ratio of the number of turns N1 of the first winding to the number of turns N2 of the second winding (i.e., the turns ratio). The relationship may be expressed mathematically as
                                          V            ⁢                                                  ⁢            1                                V            ⁢                                                  ⁢            2                          =                                                            -                I                            ⁢                                                          ⁢              2                                      I              ⁢                                                          ⁢              1                                =                                    N              ⁢                                                          ⁢              1                                      N              ⁢                                                          ⁢              2                                                          (        1        )            
An example of a transformer 200 with more than two windings is shown in FIG. 2. Such transformers are commonly used in utility line frequency applications (50/60 Hz), and in high frequency switched mode power supplies. The transformer 200 includes a first, a second and a third winding having N1, N2 and N3 turns respectively. The voltages across the first, second and third windings are V1, V2 and V3, respectively, and the currents entering the first, second and third windings are I1, I2 and I3, respectively. The transformer 200 is commonly called a series multi-winding transformer.
The relationship between voltages and currents for transformer 200 (and for other transformers having more than two windings) differs from the relationship between voltages and currents for two winding transformer (e.g., transformer 100). The voltages across all three windings of transformer 200 are related by the turns ratios in the same manner as a two winding transformer (e.g., transformer 100). Namely, the voltage relationships are governed by the equation:
                                          V            ⁢                                                  ⁢            1                                N            ⁢                                                  ⁢            1                          =                                            V              ⁢                                                          ⁢              2                                      N              ⁢                                                          ⁢              2                                =                                    V              ⁢                                                          ⁢              3                                      N              ⁢                                                          ⁢              3                                                          (        2        )            
However, the current relationship for a two winding transformer (e.g., 100) expressed in equation (1) is not valid in the case of transformer 200. Knowing the current of one of the windings and the turns ratios does not allow determination of the current of the other windings. Instead, the sum of ampere-turn products of all windings must be equal to zero. Mathematically this rule is expressed as:
                                          ∑                          k              =              1                        n                    ⁢                      Ik            *            Nk                          =        0                            (        3        )            
A parallel multi-winding transformer 300 is shown in FIG. 3. The transformer 300 includes a first, a second and a third winding having N1, N2 and N3 turns, respectively. The voltages across the first, second and third windings are V1, V2 and V3, respectively, and the currents at the beginning of the first, second and third windings are I1, I2 and I3, respectively.
Parallel multi-winding transformer 300 is characterized by a deterministic current relationship between any two windings:I1*N1=I2*N2=I3*N3  (4)However, the law for the voltages for parallel multi-winding transformer 300 reflects a weaker interrelationship given by:
                                          ∑                          k              =              1                        n                    ⁢                      Vk            Nk                          =        0                            (        5        )            
Transformer 300 may be used for power sources where output current is controlled (rather than output voltage) or where equal current distribution in multiple branches of the circuit is desired for more accurate operation or stress reduction.
The relationships presented above, e.g., equations (2)-(5), demonstrate the difference between series multi-winding transformers and parallel multi-winding transformers. These relationships do not include the effect of various non-ideal properties of the transformers, as the non-ideal properties are generally irrelevant for illustration of the differences between these two structures
One non-ideal property of transformers that is important in some applications, including, for example, high frequency applications, is leakage inductance. Leakage inductance represents energy stored in the magnetic field that is not coupled between various windings. Leakage inductance manifests itself as if an uncoupled inductor was placed in series with the transformer winding. This inductor creates additional impedance, which may interfere with the operation of the circuit.
Various techniques for constructing transformers with low leakage inductance are known. These known techniques are commonly based on physical arrangement of the core and the windings with different windings placed as close to one to another as possible. Two of the techniques for constructing transformers with low leakage inductance are interleaving and multifilar winding. In interleaving, windings are divided into multiple sections arranged in alternate layers. In multifilar winding, more than one winding is wound on a core using isolated multistrand wires.
These known techniques for constructing low leakage inductance transformers, however, are typically applicable only to series multi-winding transformers, as the techniques require different windings to be placed physically on the same part of a core. This kind of physical proximity generally may not be used for a parallel multi-winding transformer, as it is not compatible with its structure.